1. Field of the Invention
This invention relates to the preparation and properties of novel types of molecular sieve cracking catalysts consisting of cross-linked smectites. The cross-linked smectite structure includes either hydrogen or rare-earth cations and has a relatively large pore size which allows the catalysts to be used in cracking reactions of molecules having relatively large kinetic diameters such as found in heavy petroleum cuts, coalderived liquids, shale oils, and the like.
2. The Prior Art
Recent developments relating to the cost, availability and reserve depletion of worldwide stocks of petroleum has focused increasing attention on conservation and development of alternate sources of synthetic liquid and gaseous fuels from materials such as coal, oil shale, and tar sands. Likewise, attention is also being directed to better utilization of native black oils and petroleum resids. The conversion of these heavy liquids to distillate products such as gasoline usually requires catalytic processing. One of the most important and well-established catalytic processes is catalytic cracking.
The relatively recent discovery and development of molecular sieve catalysts has had a tremendous impact on the petroleum refining industry in that the conversion rates as well as products distribution were greatly improved. Accordingly, as one considers the upgrading of heavy liquid and solid feedstocks into light liquid fuels, it is possible to visualize the benefits which could be realized with molecular sieves of optimized pore dimensions and with the desired degree of catalytic activity for effective cracking of such feedstocks.
Te polymeric backbone of molecular sieve systems can be inorganic, organic or composite in nature. As a rule, such systems possess an intracrystalline pore network of well-defined geometry and critical cross-sectional sizes. Catalysis of organic reactions by molecular sieves is characterized by several specific features:
(a) Organic substrates are "intrasorbed" in the sieve channel system, i.e. due to the constraining pore size and the "concave" geometry of the internal zeolitic surface an incoming molecule is usually under the simultaneous action of an ensemble of surrounding catalytic sites. Consequently, substrate polarization is considerably stronger, that is, activation is easier, compared to that with conventional catalysts. Further, as a result of approximation and orientation effects operative in the channel systems, intrasorbed reactant molecules are in many cases, favorably juxtaposed, with consequent decrease in the activation entropy of the reaction.
(b) Incorporation of catalytically active sites or chemically reactive species in the molecular sieve framework allows for the design and synthesis of a wide variety of specific adsorbents, catalysts and polymeric reagents.
(c) The specific geometry and dimensions of the channel system in a given molecular sieve catalyst allows for performance of molecular-shape selective processes.
During the past fifteen years, interest in molecular sieve systems has been primarily concerned with the structure, properties, and application of natural and synthetic zeolites. Appropriately modified zeolites have gained importance as adsorbents in separation of industrial mixtures, or as catalysts for certain types of organic processes such as cracking, hydrocracking, isomerization, hydroisomerization, alkylation and dealkylation of simple aromatics, etc. However, it has been realized that there are certain severe limitations in the catalytic application of zeolites. In particular, due to the narrow range of critical pore sizes found in such systems (approximately 3-10 A) intrasorption and reaction of bulky or even medium-sized organic molecules is impossible. For instance, it has been demonstrated that most of the molecules present in raw coal liquids cannot penetrate into the intracrystalline pores of conventional zeolite catalysts. Furthermore, certain organic substrates, including monocyclic aromatic compounds have exhibited low intracrystalline diffusivity in zeolitic media, resulting in poor recoveries and fast catalyst aging. It is, therefore, highly desirable to prepare a new class of molecular sieve catalysts with expanded pore size which would permit admission and free diffusion of large hydrocarbon and other molecules in the intracrystalline pore system. Additionally, it would also be an advancement in the art to provide a catalyst which will combine an expanded pore size (e.g. above about 10 A) with high acidity and cracking activity.